When I go to your web site, and order a product $X, I expect that you have $X in stock to ship to me. If I wanted a backorder, there are dozens of other, cheaper vendors who can backorder $X, but who are honest enough to not when they don't have an item in stock.

Even if you have to suddenly backorder an item, tell me that you have done so, instead of waiting for me to wonder if the post office has lost it or something. If I have to ask you where it is, and you only then tell me it's backordered, I will cancel the order.